Let's say that keyword "plastic machine" has 11.000 searches per months and "plastic machinery" has 1000 searches. Let's also say www.plasticmachine.com domain is taken and that the main keyword inside the site will be "plastic machine". Would be better to register www.plastic-machine.com or www.plasticmachinery.com? Please consider only SEO factors, which site would be showed higher if all other factors were the same? I have heard that hyphenated domain name should be avoided. Will google take machinery also as machine? tnx!
If your main keyword is "Plastic Machine" then plastic-machine.com would be the best choice from an SEO perspective (but not necessarily from a user perspective). Domain names only give a major boost to rankings for a particular keyword phrase if the domain name EXACTLY matches the keyword phrase. When it's not an exact match, having a variation of the keyword in the domain is generally no more useful than having the keyword in a folder or page name in the URL. The reasons people say to avoid using hyphens in domain names is NOT because of SEO implications. It's a usability thing... It makes the exact spelling of your domain name harder to remember, harder to type, longer to type, etc.
or maybe even plasticmachine.net? so choices are plasticmachine.net, plasticmachinery.com, plastic-machine.com. Again, only from SEO perpective.